In the field of high-speed optics—reference is made to signals having very short pulses—the performance of an optical signal, emitted in the form of a train of pulses by a pulsed laser source, is likely to be influenced in particular by two factors: variations in amplitude (or power) and temporal variations of the signal.
Under these conditions, knowledge of power variations of an optical signal, of which the relationship to the mean power of the signal is generally referred to as the relative amplitude jitter or relative power jitter, is of great importance in order to characterize an ultra-short (picosecond or femtosecond) source, both in terms of quality and stability, for applications such as optical telecommunications, but also for any type of application in which the stability of the signal is a main criterion (producing a supercontinuum, metrology, etc.).
In order to determine this relative amplitude jitter, it is known to carry out processing of the optical signal, processing which consists in electronically measuring the power of at least one temporal portion of the optical signal. For this, a photodiode receives the optical signal and converts it into an electrical signal, having a pace adapted to the rate of repetition of the pulses (in the case of a pulsed signal) or to a predetermined measurement repetition rate, and this electrical signal is then directly analysed. The relative optical amplitude jitter is then transformed into the relative electrical amplitude jitter, which electrical amplitude jitter can then easily be quantified by conventional means, such as a digital sampling oscilloscope optionally provided with computer analysis tools.
By its electronic nature, said solution provides a certain flexibility in terms of data use. It makes it possible to visualise the measured jitter in a synthetic manner, for example represented in the form of an “eye diagram” for which the different measured portions of the signal are resynchronised and superposed, the thickness represented on the diagram directly indicating the range of power variation of the signal. It is also possible, owing to an electronic measurement of this type, to establish a statistical analysis of the amplitude variation of the signal from histograms, and this makes it possible to provide more precise information about the relative amplitude variation of said signal, that is to say the power probability density law.
An electronic measurement device of this type is extremely well suited to analysing an optical signal having a relative power jitter which is at least moderate (significantly higher than 20%), even significant, which is the case for strings of pulses degraded by optical transmission over a long distance. On this basis, this device is currently used for evaluating quality factors in the field of optical telecommunications.
However, this device has a major drawback when a low level of relative power jitter is being measured. This is because, as with any type of processing applied to an optical signal, said device generates a processing noise which is independent of the optical signal. In this particular case of an electronic measurement device, the photodiode and the electronics associated therewith produce relative amplitude variations of which the order of magnitude is generally between 2 and 5%. Due to the random nature of this processing noise and the relative power variation intrinsic to the optical signal, they combine in a random manner, in such a way that the initial optical jitter can be scrambled within the processing noise when said noise and said power variation have substantially similar orders of magnitude. For this reason, resorting to electronic measurement for a signal having a low relative amplitude jitter cannot be considered to be relevant.
Of course, a drawback of this type can be overcome by using an electronic device having a greater stability, for which the processing noise produced is lower, but this can only be carried out at significant extra cost.
To eliminate the influence of electronic processing noise, it is known to carry out a radio-frequency analysis of the signal, as described for example in the publication “Characterization of the noise in continuously operating mode-locked lasers” (D. Von der Linde, Appl. Phys. B 39, 201-217, 1986). In order to achieve this, an appropriate photodetector is used which is associated with an electronic spectrum analyser in order to analyse the spectral properties of a large number of harmonics of the rate of repetition of the optical signal. In this way, the spectrum analysis allows the relative power jitter which is likely to be produced by the photodetector to be compensated.
However, a measurement system of this type requires a high bandwidth photodetector. It is in particular not adapted, moreover, for trains of pulses having a high rate of repetition, typically greater than 5 GHz, unless of course excessively expensive electronic spectrum analysers are provided.
Still with the object of eliminating the influence of the electronic processing noise, another solution is that of analysing the optical signal by intensity autocorrelation, as is described for example in the publication “All-optical measurements of background, amplitude, and timing jitters for high speed pulse trains or PRBS sequences using autocorrelation function” (J. Fatome et al., Opt. Fiber. Technol., 84-91 2007) or in the patent specification US 2003/0095304. This analysis method consists essentially in establishing an optical correlation between at least two successive pulses of a pulsed optical signal, then in deriving therefrom a power probability distribution function. The use of electronics which are too large is therefore avoided.
However, a number of drawbacks result from the distinctive optical properties of this system. Said system implements polarisation components which consequently make it dependent on polarisation and which may therefore limit the potential applications thereof. Moreover, it involves a high number of mechanical parts, which proves expensive and complex to control mechanically, and it also requires the light beam to be propagated at least in part in the open air, where it is likely to be degraded, and this limits the precision of the measurements. In addition, since the correlation between two successive pulses is carried out by introducing a delay of the first pulse relative to the second, it may be necessary to provide a very long propagation space for the pulse to be delayed in the case in which the rate of repetition of the optical signal is low. As a result, this system is not adapted to trains of pulses having a low rate of repetition, typically less than 20 GHz.